Reverse Circulation rotary drilling, also known as “RC Drilling” or “Dual Wall Drilling”, has been used to sink boreholes into the ground for example, for mineral exploration.
FIG. 13 shows an exemplary device for such Reverse Circulation rotary drilling, employing a dual wall pipe 122 comprising an outer drill rod 124 with an inner rod 126 located inside said outer drill rod 124 and provided with a drill bit 140 at its distal end.
A drilling fluid, which may be high pressure air or water, is passed in the outer flow path defined between the outer and inner rods 124, 126, down to the drill bit 140. Once the drilling fluid hits said drill bit, the cuttings mixed with the said fluid are forced up to the center of the bit and back up the inner tube 126. Cuttings are so returned to the surface and collected for later use.
This kind of tooling, however, is not appropriate for grouting operations.
Injection of the grout through the annular space between the outer and inner rod is undesirable, particularly when the grout has a high viscosity and is of low mobility. Due to the large ‘wetted-perimeter’ of this annular space, there is a high risk that the grout may plug the device.
Injection of the grout through the inner rod can damage and/or destroy the drill bit. Also, the drill bit prevents free flow of grout into the drill hole, particularly when the grout is of low mobility.
Taking the inner rod and the drill bit out of the outer drill rod, to allow for the free flow of grout into the outer drill rod is time-consuming and presents problems when drilling and grouting operations must happen very quickly. Likewise, removing the dual wall pipe for introducing a separate injection device in the drill hole also necessitates much time and additional equipment.